isolated cooling system overflow tank

[sixto]

Why would a cooling system with a good radiator cap not draw coolant from the overflow tank as it loses coolant?

Thanks,
Sixto
87 300D^2

[Stretch]

Er sorry I need to ask a pedantic question!

Do you mean expansion tank or overflow tank?

An overflow tank is typically positioned below a radiator and is just used to catch fluid that has been expelled. It doesn't give it back. An expansion tank is different; it uses air trapped above fluid in a remote tank to allow for expansion or contraction within a cooling system. If fluid isn't move between the cooling system and the expansion tank I'd suspect an air lock somewhere.

[David Wilson]

The system has to be sealed well to provide the vacuum/suction needed to pull fluid back into the expansion tank when cooled. I've always thought that the very reason that fluid is lost is the reason that the fluid doesn't return from overflow.

[sixto]

Expansion tank, I guess. I wanted to be clear it's not part of the pressurized system but there's a hose to the bottom of the expansion tank so coolant is meant to be drawn back into the radiator.

I see green stuff below the heater valve which could be coolant residue. I was thinking there's a small leak in the valve slowly seeping coolant when hot then allowing in air as the system cools. I suppose a cooling system pressure check is in order but will it find a breach that only manifests when the coolant's hot?

Sixto
87 300D^2

[David Wilson]

You may be thinking it only leaks when it's hot, but in reality it's leaking only when it's under pressure. Which is a by-product of hot. A system pressure test would probably find the leak.

[retmil46]

On both 87's I've had, the original hose going from the radiator cap on the coolant tank, to the expansion/recovery tank down inside the passenger side wheel well, was old rotten and wasn't airtight. That would be the most obvious and simplest cause.

If the above hose is in good shape and the connections at both ends are airtight, next easiest candidate would be a weak/old radiator cap on the coolant tank.

If the radiator cap checks out good, well, it starts getting "fun" and expensive from there chasing down leaks.

NAPA has a rather good Mityvac coolant pressure testing kit, with various adapters for testing both the coolant system and radiator cap for most vehicles, for about $50. I picked one up about 2 or 3 months ago, and it worked quite well on both the 87 and the Jeep Liberty diesel.

[kerry]

Because it's easier to suck in air from a small leak than it is to suck in coolant from the expansion tank.

[Pooka]

Retmil has a good point. Not only could a crack in the hose he mentioned cause the problem but hose is cheap and easy to replace. I always go for the cheap and/or easy first because I usually replace everything while I am into a system anyway.

As odd as it seems the hose could have a crack and not leak coolant but when placed under a vacuum suck in air.

A system pressure test would help to find if there is a leak somewhere. Of course, everything expands when it is hot, but that is sort of built into the system. It is the increase in pressure that makes a leak appear.